It is known that asymmetric membranes can be used for the separation of ionogenic substances from, for example, aqueous solutions. The performance of such a separation, also called reverse osmosis, depends on the one hand on the nature and size of the pores, especially those of the membrane surfaces, and on the other hand on the nature and the molecular size of the ionogenic substance, i.e. the ions. The known asymmetric membranes are practically uncharged, as they do not have any ionizable groups.
Asymmetric membranes consist of a supporting main layer and of a covering layer or skin which is on one side thereof and which is usually between 1/2.mu. and 3.mu. thick. The size of the pores in the skin is important for the properties of the membrane as a separator.
Membranes with ionizable groups are also known, for example those derived from polyvinylsulfonic acids. They allow, however, even water to permeate only slowly and are permeable either for cations or anions, depending on the membrane charge, and are not asymmetric. They are used especially as ion exchange membranes in electro-dialysis.